Add support to open Arduino Libraries (=folders)

[tuxedo0801]

Is your feature request related to a problem? Please describe.
If I'm an Arduino Library deeloper, I'm not able to open a lilbrary and edit it.

Describe the solution you'd like
Opening a project filters for sketches only. Would be great if IDE allows to open folders (like VS Code allows) - maybe by switching filter from "sketch" to "folder" -

Describe alternatives you've considered
Use VS Code instead of Arduino Pro IDE :-( --> Maintain two different IDEs for one attempt: Develop Arduino stuff..

Additional context

[tuxedo0801]

Semms to be related to #101

[per1234]

Just to link related discussions together, I'm adding a reference to the discussion about adding this capability to the Arduino IDE: arduino/Arduino#3512